BuddyPress.org

This will remove the Email tab.

function waqas_remove_friends_activity_nav() {
	bp_core_remove_subnav_item( 'settings', 'notifications' );
}
add_action( 'bp_setup_nav', 'waqas_remove_friends_activity_nav', 15 );

This will set all the notification settings to ‘no’ for a specific user:

function waqas_email_notifications( $user_id ) {
	
	$settings = bp_settings_get_registered_notification_keys(); 
	
	foreach ( $settings as $setting ) {
		bp_update_user_meta( $user_id, $setting, 'no' );
	}
	
}

For new users, you could do this:

function waqas_bp_core_activated_user(  $user_id, $key, $user ) {
     waqas_email_notifications( $user_id );
}
add_action( 'bp_core_activated_user', 'waqus_bp_core_activated_user', 10, 3 );